Metoprolol is a drug belonging to the general class of compounds known as beta-blockers. Beta-blockers are beta-selective adrenoreceptor blocking agents, and include well-known commercial products such as propanolol and atenolol. Several members of this drug class are known to be useful in treatment of hypertension, angina pectoris, and myocardial infarction.
Although the mechanism of the antihypertensive effect of metoprolol (and other .beta.-blockers) is not known with certainty, a number of mechanistic possibilities have been advanced, including: the suppression of endogenous catecholamines at cardiac adrenergic neuron sites; a central effect leading to reduced sympathetic outflow to the periphery; and suppression of rennin activity.
The effectiveness of metoprolol in treatment of angina pectoris is likely to be associated with its tendency to reduce the oxygen requirements of the heart at various levels of effort. This effect results from blockage of catecholamine-induced increases in heart rate, blood pressure, and in velocity and extent of myocardial contraction.
Metropolol is regarded to be a relatively selective .beta.-blocker. That is, it has a preferential effect on .beta.1 adrenoreceptors which are predominant in cardiac muscle. This selectivity is not absolute, however, and metoprolol also exhibits activity on .beta.2 adrenoreceptors located in bronchial and peripheral vascular tissue.
Metropolol is a racemic mixture. That is, it is a mixture of optical isomers, called enantiomers. Enantiomers are structurally similar compounds which differ only in that one isomer is a configurational mirror image of the other and the mirror images cannot be superimposed. This phenomenon is known as chirality. Although structurally similar, enantiomers can have profoundly different effects in biological systems; one enantiomer is often biologically active while the other has little or no biological activity at all.